Just to clarify the terms: A practice is something you do, probably requiring some training, a standard of professionalism, and code of ethics (journalism, medicine, or construction). A medium is a vehicle through which information is transmitted and displayed (broadcast television, HTTP/HTML, newspapers). And a form is a stylistic genre, a way of organizing and presenting information or stories (haiku, sonnets, novels, magazine stories, weblogs).
So far, there’s not really a “practice” of blogging in any coherent sense, though I suppose one could emerge eventually. Peterme is right on that blogging is primarily a form.
But there is a sense in which the weblog is also a medium, and that’s syndication. … When people start using aggregators … with integrated aggregator/weblog tools, such as Radio, they can post their own responses much more quickly. That creates a kind of “virtuous circle” of conversation, as Udell has noted.
Tweney thinks that we’ll see syndication behaving more like a medium as we move further along the adoption curve.